Multi-piece solid golf ball

ABSTRACT

In a golf ball having a core, an outermost layer and an intermediate layer therebetween, the intermediate layer is formed of a thermoplastic resin composition that includes (A) an ionic olefin-methacrylic acid-unsaturated carboxylic acid ester copolymer, (B) a nonionic olefin-acrylic acid copolymer, (C) an organic acid or a metal salt thereof, and (D) a basic inorganic metal compound, and the outermost layer is formed of a thermoplastic resin composition that includes (a) a specific ionic olefin-unsaturated to carboxylic acid copolymer and (h) a specific ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer. The ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) in a dynamic viscoelasticity test on the outermost layer-forming resin composition conducted under specific conditions is not more than 0.150. This combination of intermediate layer and outermost layer materials has is a large spin rate-lowering effect on the ball, enabling the ball to travel further.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2016-234986 filed in Japan on Dec. 2, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a multi-piece solid golf ball having a core, an outermost layer, and at least one intermediate layer therebetween. The invention relates more particularly to a golf ball having excellent spin properties in which the outermost layer is formed of a composition that includes a high-acid-content ionomer.

BACKGROUND ART

It has hitherto been known that when a golf ball is hit with a club such as a driver, if the amount of spin incurred by the ball is small (low), the distance traveled by the ball increases, which is advantageous. In recent years, one technical means for achieving such a lowered spin rate has been to use an ionic composition (ionomer) as the base resin of the golf ball cover material. In particular, in golf balls having a multi-layer structure, highly neutralized ionomers wherein acid groups within the resin composition are neutralized to a high ratio have been used as the resin material (resin composition) of an inner layer. As a result, a spin rate-lowering effect has been fully imparted to the ball, which has led in turn to an increased distance.

However, it appears that highly neutralized ionomers are used as the resin material of the inner layer, and so their advantageous effects for achieving lower spin rates in golf ball are not being fully exhibited. Also, in terms of their material characteristics, they tend to be relatively brittle. As a result, when such resin materials are used, they possess also factors that give rise to secondary disadvantages such as a tendency for the ball to crack.

To improve the resin material used in the cover layer of golf balls, a number of technical disclosures published to date focus on the cover layer deformation behavior on shots taken with a driver and on the loss modulus of the cover material. For example, JP-A 2011-254974 describes art in which the outermost layer is made of a resin composition for which the ratio E″/G″ between the tensile loss modulus E″ and the shear loss modulus G″ is set to at least a given value. JP-A 2004-65409 discloses a golf ball wherein the outermost layer is formed of a polymer composition characterized by the complex modulus and loss modulus in the viscoelastic properties measured in the tension mode under a specific temperature. JP-A 2001-137386 describes a golf ball made of a core and cover wherein, for the cover material, the loss tangent (tan 6) value in the temperature dispersion curve of the dynamic viscoelasticity measured in the tension mode under specific conditions is set within a specific range.

However, in these prior-art golf balls, there is a tendency in particular for the modulus ratio and loss tangent (tan δ) of the material used for the cover (outermost layer) of the ball to become large, which signifies a decrease in the impact resilience. Even if the spin characteristics are optimized, a low impact resistance will result in an unsatisfactory ball distance. Hence, having both a good spin performance and also a good impact resilience is an important factor that determines the ball properties. Also, when an ionic thermoplastic resin is used in the outermost layer, a material having a high acid content is often selected with the aim of achieving in particular a spin rate-lowering effect. However, in such cases, depending on the compounding formulation and the hardness balance with the inner layer material, the durability of the ball sometimes worsens.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball having a multilayer structure in which, owing to synergistic effects among the cover layers, the spin rate-lowering effect is further increased and the ball durability is improved.

As a result of extensive investigations, the inventors have discovered that, in a golf ball having a core, at least one intermediate layer and an outermost layer, by forming at least one intermediate layer of a specific highly neutralized ionomeric resin material and forming the outermost layer of a material obtained by blending a high-acid-content ionic olefin-unsaturated carboxylic acid copolymer with a low/medium-acid-content ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer, a ball in which the high rebound/low spin performance is even further increased can be obtained and, moreover, the ball also has an excellent durability.

Accordingly, the invention provides a multi-piece solid golf ball having a core, an outermost layer, and at least one intermediate layer between the core and the outermost layer, wherein the intermediate layer is formed of a thermoplastic resin composition that includes:

(A) an ionic olefin-methacrylic acid-unsaturated carboxylic acid ester copolymer,

(B) a nonionic olefin-acrylic acid copolymer,

(C) an organic acid or a metal salt thereof, and

(D) a basic inorganic metal compound for neutralizing at least 80 mol % of acid groups in components (A) to (C).

Components (A) and (B) have a mixing ratio (by weight) therebetween that satisfies the condition (A):(B)=50:50 to 80:20, and this intermediate layer-forming resin composition has a Shore D hardness of from 40 to 60.

The outermost layer is formed of a thermoplastic resin composition that includes: (a) an ionic olefin-unsaturated carboxylic acid copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of at least 16 wt %; and

(b) an ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of not more than 15 wt %.

Components (a) and (b) have a mixing ratio (by weight) therebetween that satisfies the condition (a):(b)=70:30 to 90:10, and this outermost layer-forming resin composition has a Shore D hardness of at least 55 and a ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) in a dynamic viscoelasticity test conducted at a temperature of 24° C., an oscillation frequency of 15 Hz and 2.0% strain of not more than 0.150.

In a preferred embodiment of the golf ball of the invention, the core is formed of a rubber composition containing polybutadiene as the base rubber, has a diameter of not more than 40.0 mm, and has a deflection when compressed under a final load of 1.275 N (130 kgf) from an initial load state of 98 N (10 kgf) of from 3.0 to 4.5 mm.

In another preferred embodiment of the inventive golf ball, the core has a cross-sectional hardness in which the JIS-C hardness difference value obtained by subtracting the core center hardness H_(i) from the core surface hardness H₀ is at least 20.

In yet another preferred embodiment, the core has a cross-sectional hardness in which the value obtained by subtracting the JIS-C hardness at a position 5.0 mm peripherally outward from the core center (H₅₀) from the JIS-C hardness at a position 15.0 mm peripherally outward from the core center (H_(15.0)) is a positive numerical value.

In a further preferred embodiment, the core has a hysteresis loss ratio when compressed at a load cell speed of 500 mm/min and under a constant load of 5,000 N of not more than 50%.

In a still further preferred embodiment, the golf ball additionally has an envelope layer between the core and the intermediate layer.

In another preferred embodiment of the inventive golf ball, the envelope layer is formed of a thermoplastic resin composition that is other than ionic, which envelope layer-forming resin composition has a Shore D hardness of not more than 50.

In yet another preferred embodiment, the intermediate layer-forming resin composition has a ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) measured in a dynamic viscoelasticity test conducted at a temperature of 24° C. an oscillation frequency of 15 Hz and 2.0% strain, of not more than 0.110.

In still another preferred embodiment, the sum of tan δ for the outermost layer-forming resin composition and tan δ for the intermediate layer-forming resin composition, both of which are measured wider the same conditions, is not more than 0.250.

Advantageous Effects of the Invention

In the golf ball of the invention, by both using a highly neutralized ionomer in the intermediate layer material and using a composition containing a high acid-content ionomer in the outermost layer material, the viscoelastic properties of the resin materials are imparted with special characteristics. In a multilayer golf ball construction that combines such an intermediate layer material with such an outermost layer material, a large spin rate-lowering effect is obtained, resulting in an increased distance. In addition, such an arrangement confers the golf ball of the invention with an excellent durability.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent to from the following detailed description taken in conjunction with the appended diagram.

The multi-piece solid golf ball of the invention is a golf ball having a core, an outermost layer, and at least one intermediate layer between the core and the outermost layer. For example, referring to FIG. 1 which shows a schematic cross-sectional view of a golf ball according to one embodiment of the invention, the golf ball G has a core 1, an intermediate layer 2 encasing the core 1, and an outermost layer 3 encasing the intermediate layer 2. In addition, although not shown in the diagram, an envelope layer may be optionally provided between the core I and the intermediate layer 2. Also, numerous dimples D are typically formed on the surface of the outermost layer 3 in order to enhance the aerodynamic properties of the ball. Each layer of the golf ball is described in detail below.

The core is preferably formed of a rubber composition containing at least a base rubber, an unsaturated carboxylic acid and/or a metal salt thereof, and a crosslinking initiator.

It is preferable to use polybutadiene as the base rubber. The polybutadiene has a cis-1,4 bond content of at least 60% (here and below, “%” stands for percent by weight), preferably at least 80%, more preferably at least 90%, and most preferably at least 95%. Also, the polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.) of preferably at least 30, and more preferably at least 35, with the upper limit being preferably not more than 100, and more preferably not more than 90.

Illustrative examples of the polybutadiene include the following cis-1,4-polybutadiene rubbers available from JSR Corporation: high-cis BRO1, BR11, BR02, BR02L, BR02LL, BR730 and BR51.

To obtain a molded and crosslinked rubber composition having a good resilience, the polybutadiene is preferably one synthesized using a rare-earth catalyst or a nickel, cobalt or other group VIII metal compound catalyst.

The polybutadiene accounts for a proportion of the overall rubber composition which is preferably at least 40 wt %, more preferably at least 60 wt %, even more preferably at least 80 wt %, and most preferably at least 90 wt %. The polybutadiene may account for 100 wt % of the base rubber, although it preferably accounts for not more than 98 wt %, and more preferably not more than 95 wt %.

The base rubber may include rubber components other than the above polybutadiene within a range that does not detract from the advantageous effects of the invention.

Examples of such rubber components other than the foregoing polybutadiene include other polybutadienes, and diene rubbers other than polybutadiene, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

An α,β-unsaturated carboxylic acid (e.g., acrylic acid, methacrylic acid) and/or a metal salt thereof may be included as the unsaturated carboxylic acid or a metal salt thereof. Examples of the metal include zinc, sodium, potassium, magnesium, lithium and calcium. Copper is not included. α,β-Unsaturated carboxylic acids having from 3 to 8 carbon atoms are especially preferred as the α,β-unsaturated carboxylic acid. The α,β-unsaturated carboxylic acid is preferably selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, maleic acid and fumaric acid. Alternatively, a metal salt of an α,β-unsaturated carboxylic acid may be suitably used. It is especially preferable for the metal salt to be a zinc salt.

The content of the unsaturated carboxylic acid or a metal salt thereof per 100 parts by weight of the base rubber, although not particularly limited, is preferably at least 5 parts by weight, and more preferably at least 15 parts by weight, but preferably not more than 50 parts by weight, and more preferably not more than 45 parts by weight.

Suitable use may be made of, in particular, an organic peroxide as the crosslinking initiator. Illustrative examples of organic peroxides include 1,1-di(t-butylperoxy)cyclohexane, 1,1-bis-t-butylperoxy-3,3,5-trimethylcyclohexane, dicumyl peroxide, di(t-butylperoxy)-m-diisopropylbenzene and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. These organic peroxides may be of one type used alone, or two or more types may be used in combination.

The content of the organic peroxide, although not particularly limited, may be set to at least 0.1 part by weight, and preferably at least 0.3 part by weight, per 100 parts by weight of the base rubber. The upper limit may be set to not more than 5 parts by weight, and preferably not more than 2 parts by weight.

Where necessary, a commercially available antioxidant may be suitably included. For example, a compound such as 2,2′-methylenebis(4-methyl-6-tort-butylphenol) may be used. The amount of antioxidant included per 100 parts by weight of the base rubber is preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit is preferably not more than 3 parts by weight. Illustrative examples include Nocrac NS-6, Nocrac NS-5 and Nocrac NS-30 from Ouchi Shinko Chemical Industry Co., Ltd.

In addition, by directly adding water (or a water-containing material) to the rubber composition, the dissolution of organic peroxide during core compounding can be promoted. Golf balls having such a core can achieve a lower spin rate, in addition to which the durability is excellent and the change in rebound over time can be lowered. The water included in the rubber composition is not particularly limited, and may be distilled water or tap water. The use of distilled water that is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 parts by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

An organosulfur compound may be included in the rubber composition for the purpose of improving the resilience of the molded and crosslinked rubber material. Such organosulfur compounds are exemplified by thiophenols, thionaphthols, halogenated thiophenols and metals salts thereof. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol and metal salts thereof, especially zinc salts. In this case, the content of the organosulfur compound is preferably at least 0.001 part by weight, and not more than 5 parts by weight, per 100 parts by weight of the base rubber.

Various inorganic fillers may be included in the rubber composition so as to adjust the rubber weight. Illustrative examples of such inorganic fillers include zinc oxide, calcium carbonate, calcium oxide, magnesium oxide, barium sulfate and silica. The use of a metal oxide such as zinc oxide, calcium oxide or magnesium oxide is especially preferred.

The core can be obtained by vulcanizing/curing the above-described rubber composition in the same way as conventional golf ball rubber compositions. Vulcanization is carried out under conditions such as a vulcanization temperature of between 100 and 200° C. and a vulcanization time of from 5 to 40 minutes.

It is recommended that the core diameter have an upper limit of not more than 41.0 mm and a lower limit of preferably at least 25.0 mm, and more preferably at least 30.0 nun. At a diameter smaller than this, it may be difficult to obtain a sufficient spin rate-lowering effect.

It is recommended that the core (hot-molded material) have a deflection when to compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) which, although not particularly limited, is preferably at least 2.0 mm, more preferably at least 2.5 mm, and even more preferably at least 3.0 mm, with the upper limit being preferably not more than 6.0 nun, more preferably not more than 5.5 mm, and even more preferably not more than 5.0 mm. When this value is too large, even with an increased hysteresis loss for the core, the deflection of the finished ball may become large, resulting in a decreased rebound. On the other hand, when this value is too small, there is a possibility of the feel at impact becoming too hard.

The core has a center hardness expressed in terms of JIS-C hardness which, although not particularly limited, is preferably at least 40, more preferably at least 43, and even more preferably at least 45, with the upper limit being preferably not more than 70, more preferably not more than 68, and even more preferably not more than 65. At a core center hardness outside of this range, the durability may end up decreasing or it may not be possible to obtain a spin rate-lowering effect.

The core has a surface hardness expressed in terms of JIS-C hardness which, although not particularly limited, is preferably at least 60, more preferably at least 63, and even more preferably at least 65, with the upper limit being preferably not more than 95, more preferably not more than 90, and even more preferably not more than 88. At a core surface hardness lower than this range, the resilience decreases and a sufficient distance may not be obtainable. On the other hand, at a core surface hardness higher than this range, the feel at impact may become too hard or the ball durability may worsen.

The JIS-C hardness difference value obtained by subtracting the core center hardness from the core surface hardness [(core surface hardness)−(core center hardness)“, although not particularly limited, is preferably at least 10, more preferably at least 13, and even more preferably at least 15, with the upper limit being preferably not more than 45, more preferably not more than 43, and even more preferably not more than 40. At a hardness difference value that is too small, even if a good rebound can be achieved for the manufactured ball, a spin rate-lowering effect may not be obtained, as a result of which there may end up being a loss of distance. On the other hand, at a hardness difference that is too large, the ball durability markedly worsens.

In the hardness at the core interior, it is preferable for the value obtained by subtracting the JIS-C hardness at a position 5.0 mm peripherally outward from the core center (H_(5.0)) from the JIS-C hardness at a position 15.0 mm peripherally outward from the core center (H_(15.0)) to be a positive numerical value. When this H_(15.0)−H_(5.0) value is zero or negative, the core hardness gradient ceases to be one where the hardness gradually increases toward the outside. In cases such as this where either there is no hardness difference or the hardness difference is negative, either the energy that should be transferred to the core at the time of ball impact is not transferred or the energy that is transferred becomes absorbed at the core interior, as a result of which the spin rate-lowering effect that ought to be obtained may not be obtained.

In this invention, the above-mentioned center hardness and cross-sectional hardnesses at specific positions refer to hardnesses measured at the center and specific positions in the cross-section obtained by cutting the core in half through the center. The surface hardness refers here to the hardness measured at the spherical surface of the core. The JIS-C hardness refers to a hardness measured with the spring-type durometer (JIS-C model) specified in JIS K 6301-1975.

Also, the core is stipulated as having a hysteresis loss ratio when compressed at a load cell speed of 500 mm/min and under a constant load of 5,000 N of not more than 50%. When this hysteresis loss rate is 50% or more, most of the load energy incurred when the ball has been struck ends up dissipating as thermal energy. That is, the energy that should be transmitted to the ball ends up decreasing, which may lead to a decline in the rebound of the manufactured ball and an increase in the spin rate. The method of measuring hysteresis loss is carried out using, for example, a tension/compression testing machine. In the case of a core (ball), use is made of the hysteresis loss when a compressive load is applied. The test conditions are set to a load cell speed of 500 mm/min and a constant load of 5,000 N.

Next, the intermediate layer is described. The intermediate layer is formed of a thermoplastic resin composition that includes the following components (A) to (D):

(A) an ionic olefin-methacrylic acid-unsaturated carboxylic acid ester copolymer,

(B) a nonionic olefin-acrylic acid copolymer,

(C) an organic acid or a metal salt thereof, and

(D) a basic inorganic metal compound for neutralizing at least 80 mol % of acid groups in components A to C.

The olefin in component (A) is preferably one having from 2 to 6 carbon atoms, with ethylene being especially preferred. The unsaturated carboxylic acid ester in component to (A) is preferably a lower alkyl ester, and more preferably butyl acrylate or n-butyl acrylate. Moreover, in component (A), the type of metal salt used to neutralize the acid groups may be one that includes a monovalent to trivalent inorganic metal species, examples of which include zinc, sodium, magnesium, potassium and calcium. The use of zinc or sodium is especially preferred.

The olefin in component (B) is preferably an olefin having from 2 to 6 carbon atoms, with ethylene being especially preferred.

The unsaturated carboxylic acid content (acid content) in components (A) and (B) has an upper limit of preferably not more than 15 wt % and a lower limit of preferably at least 8 wt %. When the acid content is low, a resin material having a high resilience may not be obtained. On the other hand, when the acid content is high, the hardness of the resin material may become extremely high.

Components (A) and (B) are each exemplified by the product series available under the trade names ESCOR and IOTEK from ExxonMobil, and the product series available under the trade names Nucrel and Surlyn from DuPont-Mitsui Polychemicals Co., Ltd.

The mixing ratio (by weight) between components (A) and (B) satisfies the conditions (A):(B)=50:50 to 80:20, and is preferably from 55:45 to 75:25. Outside of this mixing ratio, undesirable effects such as a decrease in the ball durability and a worsening in the moldability may arise.

Component (C) is an organic acid or a metal salt thereof. Although the type thereof is not particularly limited, the use of a metal stearate or a metal oleate is especially preferred. Examples of metal stearates include magnesium stearate, calcium stearate, zinc stearate and sodium stearate. Of these, the use of magnesium stearate is especially preferred.

Component (D) is a basic inorganic metal compound, the metal species of which is exemplified by Na⁺, K⁺, Li⁺; Zn²⁺, Ca²⁺, Mg²⁺, Cu²⁺ and Co²⁺. Of these, Na⁺, Zn²⁺, Ca²⁺ and Mg²⁺ are preferred, and Mg²⁺ is more preferred. Formates, acetates, nitrates, carbonates, bicarbonates, oxides, hydroxides or the like may be used as these metal salts, and may be introduced into the resin composition.

Component (D) is included for the purpose of neutralizing at least 80 mol % of the acid groups in above components (A) to (C), and may be included in an amount suitable for neutralizing preferably at least 83 mol %, and more preferably at least 85 mol %, of the acid groups. The specific content, per 100 parts by weight of the component (A) and (B) resins, is from 0.5 to 4.0 parts by weight, and preferably from 0.75 to 3.5 parts by weight.

Optional additives may be suitably included, according to the intended use, in the resin compositions containing above components (A) to (D). Various types of additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added.

The method used to prepare the resin composition containing components (A) to (D) may be a known mixing method and is not particularly limited, although preferred use can be made of a method in which mixing is carried out using an extruder. In this case, the extruder used may be either a single-screw extruder or a twin-screw extruder, although a twin-screw extruder is more preferred because of its high kneading effect. It is also possible to use a plurality of extruders in a tandem arrangement, examples of which include two-stage tandem extruders such as a single-screw extruder/twin-screw extruder and a twin-screw extruder/twin-screw extruder.

The resin composition obtained using these components (A) to (D) has a material hardness, in terms of Shore D hardness, which may be set to at least 35, preferably at least 40, and more preferably at least 45. The upper limit, in terms of Shore D hardness, may be set to not more than 65, preferably not more than 60, and more preferably not more than 55. When this material hardness is too low, the spin rate on full shots rises excessively and a good distance may not be obtained, or the durability may worsen. When the material hardness is too high, the durability to cracking on repeated impact may worsen, or the spin rate on full shots may be too high, as a result of which a good distance may not be obtained.

The material hardness mentioned above is the hardness obtained by molding the material to be measured into a sheet of a given thickness as the test piece, and carrying out measurement using a type D durometer in accordance with ASTM D2240 (the same applies below to the material hardnesses of the subsequently described envelope layer and outermost layer).

The intermediate layer has a thickness which, although not particularly limited, is set to preferably at least 0.5 mm, more preferably at least 0.8 mm, and even more preferably at least 1.0 mm. The upper limit is set to preferably not more than 2.0 mm, more preferably not more than 1.8 mm, and even more preferably not more than 1.5 mm. When the intermediate layer is too thin, the durability may worsen. On the other hand, when it is too thick, the spin rate on full shots may rise, as a result of which a good distance may not be obtained.

The construction of the intermediate layer is not limited to one layer; two or more intermediate layers of the same or different types may be optionally formed within the foregoing range.

An envelope layer may be optionally provided between the core and the intermediate layer. In such a case, the envelope layer is directly formed over the core. The envelope layer is described in detail below.

A known resin may be used as the envelope layer-forming material. This material is not particularly limited, although formation of the envelope layer with a thermoplastic resin composition other than one that is ionic is especially preferred. The use of one, two or more selected from the group consisting of urethane, amide, ester, olefin and styrene-type thermoplastic elastomers is more preferred. The use of a thermoplastic polyether ester elastomer is most preferred because a high resilience in the desired hardness range can be obtained.

The material hardness of the envelope layer is not particularly limited, although this may be set to, in terms of Shore D hardness, preferably not more than 50, and more preferably not more than 45. The lower limit in the material hardness of the envelope layer may be set to, in terms of Shore D hardness, typically at least 15, preferably at least 20, and more preferably at least 25.

The envelope layer has a thickness which, although not particularly limited, is set to preferably at least 0.5 mm, and more preferably at least 0.8 mm. The upper limit is set to preferably not more than 2.0 mm, and more preferably not more than 1.5 mm. When the envelope layer is too thin, the ball durability may become poor or the feel at impact may worsen. When the envelope layer is too thick, the spin rate on full shots may rise, as a result of which a good distance may not be obtained.

Next, the outermost layer used in the invention is described. The outermost layer is formed of a thermoplastic resin composition that includes:

(a) an ionic olefin-unsaturated carboxylic acid copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of at least 16 wt %, and

(b) an ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of not more than 15 wt %.

Component (a) is an ionic olefin-unsaturated carboxylic acid copolymer. The olefin in component (a) is preferably one having from 2 to 6 carbon atoms, with ethylene being especially preferred. The unsaturated carboxylic acid in component (a) is preferably acrylic acid or methacrylic acid, with methacrylic acid being more preferred. The unsaturated carboxylic acid content (acid content) in component (a) has an upper limit of preferably not more than 25 wt %, and a lower limit of preferably at least 16 wt %. When this acid content is low, the ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) becomes large and a spin rate-lowering effect is difficult to obtain. On the other hand, when this acid content is high, the hardness of the resin material becomes extremely high, making it difficult to obtain the desired material hardness.

Component (a) has a weight-average molecular weight (Mw) of from 40,000 to 200,000, and preferably from 42,000 to 200,000. The molecular weight distribution of component (a), i.e., the [weight-average molecular weight (Mw)/number-average molecular weight (Mn)] value, is from 4.0 to 10.0, and preferably from 4.0 to 8.5.

Component (b) is an ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer. The olefin in component (b) is preferably one having from 2 to 6 carbon atoms, with ethylene being especially preferred. The unsaturated carboxylic acid in component (b) is preferably acrylic acid or methacrylic acid, with methacrylic acid being more preferred. The unsaturated carboxylic acid ester in component (h) is preferably a lower alkyl ester, with butyl acrylate or n-butyl acrylate being especially preferred. The unsaturated carboxylic acid content (acid content) in component (b) has an upper limit of preferably not more than 15 wt % and a lower limit of preferably at least 5 wt %,

Component (b) has a weight-average molecular weight (Mw) of from 40,000 to 200,000, and preferably from 42,000 to 200,000. The molecular weight distribution of component (b), i.e., the [weight-average molecular weight (Mw)/number-average molecular weight (Mn)] value, is from 4.0 to 10.0, and preferably from 4.0 to 8.5.

Components (a) and (b) are both ionic resins. In these components, the type of metal salt used to neutralize the acid groups should be one that includes a monovalent to trivalent inorganic metal species, examples of Which include zinc, sodium, magnesium, potassium and calcium. The use of zinc or sodium is especially preferred.

Components (a) and (b) are each exemplified by the product series available under the trade name IOTEK from ExxonMobil, and the product series available under the trade name Surlyn from DuPont-Mitsui Polychemicals Co., Ltd.

The mixing ratio (by weight) between components (a) and (b) satisfies the conditions (a):(b)=70:30 to 90:10, and is preferably from 70:30 to 85:15. Outside of this mixing ratio between components (a) and (h), cracking of the cover may arise or a spin rate-lowering effect may not be obtained.

In the resin composition containing components (a) and (b), it is critical for the ratio between the tensile storage modulus (E′) and the tensile loss modulus (E″) measured in a dynamic viscoelasticity test conducted at a temperature of 24° C., an oscillation frequency of 15 Hz and 2.0% strain, which ratio is referred to in this Specification as “tan δ,” to be not more than 0.150. Such a material both has the hardness of a material ideal for use as the outermost layer of a golf ball and also has a molecular structure that efficiently converts, without loss, energy that has been applied to the material into rebound forces. A larger spin rate-lowering effect and higher rebound performance can be imparted to the ball through these actions. An apparatus such as the EPLEXOR 500N (from GABO) can be used to carry out the dynamic viscoelasticity test.

Optional additives may be suitably included in the resin composition containing components (a) and (h), depending on the intended use thereof. In the same way as described above with regard to the intermediate layer material, various types of additives may be added to this resin composition. Likewise, the method of preparing the resin composition containing components (a) and (b) is the same as that described above with regard to the intermediate layer material.

The material hardness of the resin composition obtained using these components (a) and (b), expressed in terms of Shore D hardness, is at least 55, preferably at least 57, and more preferably at least 60. The upper limit in the material hardness of the outermost layer, in terms of Shore D hardness, is set to preferably not more than 75, and more preferably not more than 70. When the material hardness of the outermost layer is too low, the ball may be too receptive to spin or the resilience may be inadequate, as a result of which the distance may end up falling. On the other hand, when the material hardness of the to outermost layer is too high, the durability to cracking may worsen.

A known method may be employed to form the outermost layer over the intermediate layer using the foregoing outermost layer material. For example, suitable use can be made of a method that involves placing the sphere obtained by forming an intermediate layer over the core in a given injection mold, and injecting the outermost layer-forming material over the sphere; or a method that involves enclosing the sphere within a pair of half cups that have been pre-molded into hemispherical shapes, and then molding under applied heat and pressure.

The outermost layer has a thickness which, although not particularly limited, is set to preferably at least 0.5 mm, more preferably at least 0.7 mm, and even more preferably at least 1.0 mm. The upper limit is set to preferably not more than 2.0 nun, more preferably not more than 1.7 mm, and even more preferably not more than 1.5 mm. When the outermost layer is too thin, the durability may worsen. On the other hand, when it is too thick, the spin rate on shots with a W#1 may become too high, as a result of which a good distance may not be achieved.

Thus, the multi-piece solid golf ball of the invention that combines an intermediate layer formed of the above-described intermediate layer-forming resin material with an outermost layer formed of the above-described outermost layer material has an excellent rebound and a large spin rate-lowering effect, making it possible to impart an increased distance and also enabling a good durability to be obtained.

In this invention, although not particularly limited, to improve the aerodynamic properties and further increase the distance, numerous dimples may be formed on the surface of the outermost layer in the same way as in conventional golf balls. By optimizing such dimple parameters as the number of dimples types and the total number of dimples, it is possible to provide the ball with a more stable trajectory and an excellent distance performance. Moreover, in order to enhance the design features and durability of the golf ball, the outermost layer may be subjected to various types of treatment, such as surface preparation, stamping and painting.

The golf ball of the invention can be made to conform to the Rules of Golf for play. Specifically, the ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably from 45.0 to 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 3, Comparative Examples 1 to 9

Using the core-forming composition made primarily of polybutadiene that is shown in Table 1 below and is common to all the Examples, cores having a diameter of 35.2 mm (common to all the Examples) were produced by 15 minutes of vulcanization at 155° C.

TABLE 1 Core formulation (pbw) Polybutadiene 100 Zinc acrylate 34.5 Organic peroxide 1.0 Distilled water 1.0 Antioxidant 0.1 Barium sulfate 17.0 Zinc oxide 4 Zinc salt of pentachlorothiophenol 0.3 Vulcanization conditions Temperature (° C.) 155 Time (min) 15

Details on the above formulation are given below.

Polybutadiene rubber: Available under the trade name “BR 51” from JSR Corporation Zinc acrylate: Available from Nippon Shokubai Co., Ltd. Organic peroxide: Dicumyl peroxide, available under the trade name “Percumyl D” from NOF Corporation Distilled water: Available from Wako Pure Chemical industries, Ltd. Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical Industry Co., Ltd. Barium sulfate: Available under the trade name “Barico #100” from Hakusui Tech Zinc oxide: Available as “Zinc Oxide Grade 3” from Sakai Chemical Co., Ltd. Zinc salt of: Available from Wako Pure Chemical pentachlorothiophenol Industries, Ltd.

The deformation under a given load, hardness distribution and hysteresis loss ratio (%) for each of the cores produced above were evaluated. Those results are shown in Tables 3 and 4.

Core Deformation under Given Load:

This value is the deformation measured when the core was compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf). The core deformations shown in Tables 3 and 4 are each average values for 30 cores.

Core Hardness Distribution:

The core surface is a spherical surface. The indenter of a durometer was set substantially perpendicular to this spherical surface, and the core surface hardness H₀ was measured as the JIS-C hardness in accordance with JIS K6301-1975. The cross-sectional hardnesses at the core center H₁ and specific positions H_(5.0) and H_(15.0) were measured by hemispherically cutting the core in such a way as to make the cross-section a flat plane, and pushing the indenter of a durometer perpendicularly against the cross-section at the places of measurement. The results are shown as JIS-C hardness values.

Hysteresis Loss Ratio (%) of Core:

Using the tension/compression testing machine available from A&D Company, Ltd. under the product name Tensilon RTG-1310, the core was set in a compression load measurement jig and the hysteresis loss ratio (%) of the core in each Example was determined at a load cell speed of 500 mm/min and under a constant load of 5,000 N. The number of measurements for a single core was set to 5 (N=5). The averages of these measured values for each core are shown in Tables 3 and 4.

Next, using the three cover layer (envelope layer, intermediate layer and outermost to layer) materials having the properties shown in Table 2 below, an envelope layer, an intermediate layer and an outermost layer were successively injection-molded over the core to respective thicknesses of 1.2 mm, 1.3 mm and 1.25 mm, thereby giving a multi-piece solid golf ball having a four-layer construction. At this time, in all of the Working Examples and Comparative Examples, dimples in a given pattern common to each ball were formed on the surface of the outermost layer of the ball.

TABLE 2 Enve- lope layer Intermediate layer Outermost layer No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 No. 10 No. 11 No. 12 No. 13 Formulation Hytrel 3046 100 (pbw) Surlyn 9320 70 10 50 30 35 AN4221C 30 90 50 70 Magnesium 1.1 2.0 1.4 1.7 oxide Magnesium 60 60 60 60 stearate Himilan 50 50 50 20 1605 Himilan 50 AM7329 Himilan 75 65 85 50 AM7318 Himilan 25 15 50 AM7327 Himilan 50 50 1706 Himilan 80 1855 Material hardness 27.0 51.7 57.3 62.0 62.0 62.0 64.0 62.3 62.3 62.0 58.0 64.0 59.0 (Shore D) Loss tangent — 0.103 0.111 0.105 0.109 0.090 0.150 0.143 0.156 0.090 0.140 0.141 0.151 (tan δ)

Details on the resin materials in the above table are given below.

Hytrel 3046: A polyester elastomer from DuPont-Toray Co., Ltd. Surlyn 9320: An ionomer resin (methacrylic acid content, 9.6 wt %; ester content, 23 wt %; neutralizing metal, Zn) from E.I. DuPont de Nemours & Co. AN4221C: A nonionic ethylene-acrylic acid copolymer (acrylic acid content, 12 wt %) from DuPont-Mitsui Polychemicals Co., Ltd. Magnesium “Kyowamag MF 150” from Kyowa Chemical oxide: Industry Co., Ltd. Magnesium “Magnesium Stearate G” from NOF Corporation stearate:

In the above table, the Himilan series are ionomer resins from DuPont-Mitsui Polychemicals Co., Ltd. Details on the respective grades are as follows.

Himilan Methacrylic acid content, 15 wt %; neutralizing metal, Na 1605: Himilan Methacrylic acid content, 15 wt %; neutralizing metal, Zn AM7329: Himilan Methacrylic acid content, 18 wt %; neutralizing metal, Na AM7318: Himilan Methacrylic acid content, 7.0 wt %; ester content, 16 wt %; AM7327: neutralizing metal, Zn Himilan Methacrylic acid content, 15 wt %; neutralizing metal, Zn 1706: Himilan Methacrylic acid content, 10 wt %; ester content, 10 wt %; 1855: neutralizing metal, Zn

The loss tangents (Tan δ) for the above resin materials were measured with the EPLEXOR 500N (from GABO) in the tension mode, at a temperature of 24° C., an oscillation frequency of 15 Hz, and a strain of 2.0%.

The properties (initial velocity, spin rate, durability) of the golf balls obtained in these Working Examples and Comparative Examples were evaluated as described below. The results are presented in Tables 3 and 4.

Evaluation of Ball Properties Ball Deflection (mm):

The deformation (mm) of the golf ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Spin Rate of Ball (rpm):

A driver (W#1) was mounted on a golf swing robot, and the spin rate of the ball immediately after being struck at a head speed (HS) of 45 m/s was measured with an apparatus for measuring the initial conditions. The driver used was the PHYZ III (2014 model; loft angle, 10.5°) manufactured by Bridgestone Sports Co., Ltd.

Ball Durability:

The durability of the golf ball was evaluated using the ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester fires a golf ball pneumatically and causes it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates was set to 43 m/s. The number of shots required for the golf ball to crack was measured. The durability index in each Example was calculated relative to an arbitrary index of 100 for the average number of shots at which five balls (n=5) in Working Example 1 began to crack, and the durability was rated according to the following criteria.

Rating Criteria:

Excellent: Durability index was 90 or more Good: Durability index was at least 70 but less than 90 NG: Durability index was less than 70

TABLE 3 Working Example Comparative Example 1 2 3 1 2 3 Ball Core A A A A A A construction Envelope layer No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 Intermediate layer No. 2 No. 2 No. 4 No. 2 No. 3 No. 3 Outermost layer No. 8  No. 12 No. 8 No. 7 No. 7 No. 8 Core Diameter (mm) 35.2 35.2 35.2 35.2 35.2 35.2 Deformation (mm) 4.0 4.0 4.0 4.0 4.0 4.0 Core hardness H_(i) 57 57 57 57 57 57 distribution H_(5.0) 60 60 60 60 60 60 (JIS-C hardness) H_(15.0) 79 79 79 79 79 79 H₀ 84 84 84 84 84 84 H₀ − H_(i) 27 27 27 27 27 27 Hysteresis loss ratio (%) 49 49 49 49 49 49 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.6 45.5 45.6 45.6 45.6 45.5 Deformation (mm) 3.3 3.4 3.2 3.2 3.1 3.2 Evaluation Initial velocity (m/s) 77.23 77.20 77.17 77.26 77.20 77.40 results Spin rate (rpm) 2,720 2,381 2,621 2,878 2,810 2,876 Durability Excellent Excellent Excellent Good Good Excellent Tan δ Intermediate layer 0.103 0.103 0.105 0.103 0.111 0.111 Cover 0.143 0.141 0.143 0.150 0.150 0.143 Tan δ (intermediate layer + cover) 0.246 0.244 0.248 0.253 0.261 0.254

TABLE 4 Comparative Example 4 5 6 7 8 9 Ball Core A A A A A A construction Envelope layer No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 Intermediate layer No. 2 No. 2 No. 2 No. 6 No. 2 No. 5 Outermost layer No. 9  No. 10  No. 11 No. 8  No. 13  No. 13 Core Diameter (mm) 35.2 35.2 35.2 35.2 35.2 35.2 Deformation (mm) 4.0 4.0 4.0 4.0 4.0 4.0 Core hardness H_(i) 57 57 57 57 57 57 distribution H_(5.0) 60 60 60 60 60 60 (JIS-C hardness) H_(15.0) 79 79 79 79 79 79 H₀ 84 84 84 84 84 84 H₀ − H_(i) 27 27 27 27 27 27 Hysteresis loss ratio (%) 49 49 49 49 49 49 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.7 45.6 45.6 45.4 45.5 45.5 Deformation (mm) 3.3 3.2 3.3 3.1 3.3 3.3 Evaluation Initial velocity (m/s) 77.10 77.25 77.22 77.33 77.30 77.25 results Spin rate (rpm) 2,793 2.885 2,980 2,716 2,828 3,011 Durability Excellent Good Excellent NG Excellent Excellent Tan δ Intermediate layer 0.103 0.103 0.103 0.090 0.103 0.109 Cover 0.156 0.090 0.140 0.143 0.151 0.151 Tan δ (intermediate layer + cover) 0.259 0.193 0.243 0.233 0.254 0.260

The results in above Tables 3 and 4 demonstrate that the golf balls according to the Working Examples of the invention each achieved a reduced spin rate on shots with a driver, and moreover had an excellent initial velocity and durability.

By contrast, in Comparative Example 1, the outermost layer lacked component (a and so the spin rate was high.

In Comparative Example 2, the mixing ratio between components (A) and (B) in the intermediate layer was unsuitable and the outermost layer lacked component (a), as a result of which the spin rate was high.

In Comparative Example 3, the mixing ratio between components (A) and (B) in to the intermediate layer was unsuitable, and so the spin rate was high.

In Comparative Example 4, because the mixing ratio between components (a) and (b) in the outermost layer was unsuitable and the tan δ was outside of the proper range, the initial velocity was somewhat low and the spin rate was somewhat high.

In Comparative Example 5, the outermost layer lacked component (a), and so the spin rate was high.

In Comparative Example 6, the outermost layer lacked component (a), and so the spin rate was high.

In Comparative Example 7, compounding in the intermediate layer was unsuitable and the hardness was high, as a result of which the durability was poor.

In Comparative Example 8, because the mixing ratio between components (a) and (b) in the outermost layer was unsuitable and the tan δ was outside of the proper range, the spin rate was high.

In Comparative Example 9, because the mixing ratio between components (A) and (B) in the intermediate layer was unsuitable and the tan δ was outside of the proper range, the spin rate was high.

Japanese Patent Application No, 2016-234986 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an outermost layer, and at least one intermediate layer between the core and the outermost layer, wherein the intermediate layer is formed of a thermoplastic resin composition comprising: (A) an ionic olefin-methacrylic acid-unsaturated carboxylic acid ester copolymer, (B) a nonionic olefin-acrylic acid copolymer, (C) an organic acid or a metal salt thereof, and (D) a basic inorganic metal compound for neutralizing at least 80 mol % of acid to groups in components (A) to (C), components (A) and (B) having a mixing ratio (by weight) therebetween that satisfies the condition (A):(B)=50:50 to 80:20, which first resin composition has a Shore D hardness of from 40 to 60; and the outermost layer is formed of a thermoplastic resin composition comprising: (a) an ionic olefin-unsaturated carboxylic acid copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of at least 16 wt %, and (b) an ionic olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester copolymer having a weight-average molecular weight (Mw) of from 40,000 to 200,000, a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio (Mw/Mn) of from 4.0 to 10.0, and an unsaturated carboxylic acid content of not more than 15 wt %, components (a) and (b) having a mixing ratio (by weight) therebetween that satisfies the condition (a):(b)=70:30 to 90:10, which outermost layer-forming resin composition has a Shore D hardness of at least 55 and a ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) in a dynamic viscoelasticity test conducted at a temperature of 24° C., an oscillation frequency of 15 Hz and 2.0% strain of not more than 0.150.
 2. The golf ball of claim 1, wherein the core is formed of a rubber composition containing polybutadiene as the base rubber, has a diameter of not more than 40.0 mm, and. has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of from 3.0 to 4.5 mm.
 3. The golf ball of claim 1, wherein the core has a cross-sectional hardness in which the JIS-C hardness difference value obtained by subtracting the core center hardness H_(i) from the core surface hardness H₀ is at least
 20. 4. The golf ball of claim 1, wherein the core has a cross-sectional hardness in which the value obtained by subtracting the JIS-C hardness at a position 5.0 mm peripherally to outward from the core center (H_(5.0)) from the JIS-C hardness at a position 15.0 mm peripherally outward from the core center (H_(15.0)) is a positive numerical value.
 5. The ball of claim 1, wherein the core has a hysteresis loss ratio When compressed at a load cell speed of 500 mm/min and under a constant load of 5,000 N of more than 50%.
 6. The golf ball of claim 1, further comprising an envelope layer between the core and the intermediate layer.
 7. The golf ball of claim 1, wherein the envelope layer is formed of a thermoplastic resin composition that is other than ionic, which envelope layer-forming resin composition has a Shore D hardness of not more than
 50. 8. The golf ball of claim 1, wherein the intermediate layer-forming resin composition has a ratio (tan δ) between the tensile storage modulus (E′) and the tensile loss modulus (E″) measured in a dynamic viscoelasticity test conducted at a temperature of 24° C., an oscillation frequency of 15 Hz and 2.0% strain of not more than 0.110.
 9. The golf ball of claim 1, wherein the sum of tan δ for the outermost layer-forming resin composition and tan δ for the intermediate layer-forming resin composition, both of which are measured under the same conditions, is not more than 0.250. 